-LRB- CNN -RRB- Nineteen-year-old Tony Robinson was not armed when a Madison , Wisconsin , police officer fatally shot him , Police Chief Mike Koval said Saturday .

`` We have to be clear about this , '' Koval said . `` He was unarmed . That 's going to make this all the more complicated for the investigators , the public , to accept , to understand ... why deadly force had to be used . ''

The officer who killed Robinson , 12-year department veteran Matt Kenny , had used deadly force before , shooting and killing a man in 2007 , the chief said .

Kenny was exonerated of any wrongdoing and even received a commendation , the chief said , adding that the incident was `` concluded to be a suicide by cop '' situation .

Robinson was shot after allegedly assaulting Kenny , who was responding Friday evening to a reported disturbance at a Madison residence , Koval said .

Afterward , protesters in this university town took to the streets and converged on City Hall , chanting `` Black lives matter . ''

Koval called for calm , while acknowledging the protests are reminiscent of those that followed the deaths of black men in Ferguson , Missouri , and Staten Island , New York , after confrontations with police .

Police described Robinson as African-American .

`` In light of so much things that have happened not just across the country , but in our own community , it 's understandable that the reaction at the scene and of some of our citizens is extremely volatile , emotional and upsetting , '' the police chief told CNN affiliate WKOW-TV . `` And we understand that . That 's absolutely appropriate under these circumstances . We would urge , obviously , that everyone exercise restraint . ''

Koval said he visited the home of Robinson 's mother late Friday night and met Robinson 's grandparents outside the house .

They talked and prayed , the chief said , but they advised him to put off visiting Robinson 's mother `` based on the dynamics '' of the situation .

`` I could n't even begin to get my hands around the enormity of the loss and the tragic consequences , '' he said . `` Nineteen years old is too young . ''

Robinson 's mother , Andrea Irwin , told CNN affiliate WKOW that she did n't understand what happened . `` My son has never been a violent person , never , '' she said . `` To die in such a violent way baffles me . ''

Irwin said Robinson served as a father figure to her other children .

`` He was our caretaker and so gentle , '' she said .

The incident started when authorities got a call that a black male was yelling and jumping in front of cars , Koval said . Dispatchers identified him as Robinson , according to 911 audio obtained by WKOW .

A little later the dispatcher says , `` Apparently Tony hit one of his friends , um no weapons seen . ''

About four minutes later , the dispatcher says , `` I got another call for the same suspect at -LSB- the same address -RSB- . He tried to strangle another patron . ''

About 30 seconds later , an unidentified officer says , `` Shots fired , shots fired . ''

When Kenny went to the apartment , he heard some commotion and forced his way in , Koval said .

`` Once inside the home the subject involved in this incident -- the same one allegedly out in traffic and that had battered someone -- assaulted my officer , '' Koval said .

After that , according to the chief , `` The officer did draw his revolver and subsequently shot the subject . ''

Backup officers and others at the scene performed CPR on the young man , who later died at the hospital .

Kenny suffered a blow to the head , but is being treated and will be released , Koval said . Kenny has been placed on administrative leave with pay .

Koval said he 's not sure what Robinson was doing at the house in the first place .

`` His relationship to the home is unclear to me , although there were certainly familiar acquaintances . This was not a random place . He had hung out . ''

In a statement Saturday , state Attorney General Brad Schimel said he `` can only imagine the heartbreak '' of Robinson 's parents and added he 's `` concerned for the officer ... who , I imagine , is experiencing great trauma as well . ''

`` They are all in my thoughts and prayers , '' Schimel said .

Under Wisconsin law , officer-involved shootings are investigated by an outside agency , in this case the Division of Criminal Investigation . Once DCI completes the investigation , the report will go to the local district attorney , Koval said .

Some are demanding answers sooner rather than later .

On Friday night , dozens of demonstrators came out to the area around the apartment , which police had blocked off . A group also moved toward City Hall before dispersing early Saturday .

`` Who do we trust ? '' some called out , prompting the response , `` No one ! ''

And in another refrain , they chanted , `` Black lives matter . ''

The protesters ' sentiments were echoed online , where some adopted the #WillyStreet hashtag in reference to Williamson Street , where the shooting happened .

`` Praying for Madison tonight , '' wrote one activist . `` Stand up , sit in , walk out - until u get answers . And until there are no more hashtag eulogies . ''

Mayor Paul Soglin spoke to the raw feelings , calling what happened `` an enormous tragedy . ''

`` We 've got a family that 's really hurting , '' Soglin said , according to WKOW . `` And we 've got a city and neighborhood that 's feeling pretty well hurt itself . ''

Some of the protesters were members of the Young , Gifted and Black Coalition that was formed last summer after the Michael Brown shooting in Ferguson .

The group wants more reactive policing in African-American neighborhoods , said group member Brandi Grayson .

She said Madison police park on street corners in African-American neighborhoods and wait for something to happen , which leads to residents being hassled . That does n't happen in white neighborhoods , she said .

She said Young , Gifted and Black will be working with church groups on Sunday and will hold more rallies Tuesday and Wednesday .

CNN 's Faith Karami and Kristina Sgueglia contributed to this report .

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Tony Robinson , 19 , was unarmed when an officer killed him , police chief says

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Officer has been involved in a fatal shooting on duty in 2007 , chief says

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Activists hit the streets and take to Twitter to vent anger over the shooting